The subject matter disclosed herein relates to an MRI (Magnetic Resonance Imaging) apparatus and a control method therefor, and more particularly to an MRI apparatus which performs MR imaging in an SSFP (Steady State Free Precession) pulse sequence in which RF (radio frequency) excitation is accomplished by a phase cycling method and a control method therefor.
As one of the methods for performing MR imaging of a subject with an MRI apparatus, there is an imaging method using an SSFP pulse sequence in which RF excitation is accomplished by a phase cycling method. By this method, the SSFP pulse sequence is executed while varying the phase of RF excitation by a prescribed step per TR (repetition time). A plurality of series of phase steps are made available, and the step series is changed over from one to another every round of data collection, namely at every Nex (number of exposure). And an image is reconstructed from collected in each round, and images free from band artifacts are obtained by subjecting the images to total addition, averaging, MIP (maximum intensity projection), RMS (root mean square) and the like (see, for example, U.S. Patent Publication No. 2006/0088083).
The imaging method described above, since it requires repetition of data collection often as the number of phase step series, takes a long time to accomplish imaging.
It is desirable that a problem described previously is solved.
In a first aspect, the invention provides an MRI apparatus having an imaging device which performs MR imaging in an SSFP pulse sequence in which RF excitation is accomplished by a phase cycling method and a control device which controls it, the MRI apparatus being characterized in that the control device causes the imaging device to collect data regarding all the frequency regions in a k-space in an SSFP pulse sequence in a first phase series out of a plurality of phase series; to collect data regarding low frequency regions in the k-space in the SSFP pulse sequence in the remaining phase series out of the plurality of phase series; to reconstruct an image by Fourier transform of the data regarding all the frequency regions collected in the SSFP pulse sequence in the first phase series; to generate frequency data of the k-space by inverse Fourier transform of the image; to separate the frequency data into data of the low frequency regions in the k-space and data of high frequency regions; to add data whose value is 0 to the separated data of the low frequency regions as substitute for the data of the high frequency regions; to add data whose value is 0 to the separated data of the high frequency regions as substitute for the data of the low frequency regions; to add data whose value is 0 to the data of the low frequency regions collected in the SSFP pulse sequence in the remaining phase series as substitute for the data of the high frequency regions; to reconstruct low space frequency images based on the low frequency regions to which the substitute data have been added; to reconstruct high space frequency images based on the high frequency regions to which the substitute data have been added; and to totally add the low space frequency images and the high space frequency images multiplied by a scaling factor.
In a second aspect, the invention provides a version of the MRI apparatus according to the first aspect, characterized in that the scaling factor is the ratio between the sum of a plurality of factors, figured out for each of a plurality of images reconstructed as the root-mean square of pixel values on the basis of the data of the low frequency regions to which the substitute data have been added, and one factor out of those factors.
In a third aspect, the invention provides a version of the MRI apparatus according to the second aspect, characterized in that the one factor has been figured out of an image which, out of the plurality of images, derives from data collected in the SSFP pulse sequence in the first phase series.
In a fourth aspect, the invention provides a version of the MRI apparatus according to the first aspect, characterized in that in the low frequency regions, the matrix size in the central part, of the k-space having a matrix size of 256×256, is 64×256.
In a fifth aspect, the invention provides a version of the MRI apparatus according to the first aspect, characterized in that the plurality of phase series are four phase series.
In a sixth aspect, the invention provides a method for controlling an MRI apparatus which performs MR imaging in an SSFP pulse sequence in which RF excitation is accomplished by a phase cycling method, the MRI apparatus control method being characterized in that the MRI apparatus is caused to collect data regarding all the frequency regions in a k-space in an SSFP pulse sequence in a first phase series out of a plurality of phase series; to collect data regarding low frequency regions in the k-space in the SSFP pulse sequence in the remaining phase series out of the plurality of phase series; to reconstruct an image by Fourier transform of the data regarding all the frequency regions collected in the SSFP pulse sequence in the first phase series; to generate frequency data of the k-space by inverse Fourier transform of the image; to separate the frequency data into data of the low frequency regions in the k-space and data of high frequency regions; to add data whose value is 0 to the separated data of the low frequency regions as substitute for the data of the high frequency regions; to add data whose value is 0 to the separated data of the high frequency regions as substitute for the data of the low frequency regions; to add data whose value is 0 to the data of the low frequency regions collected in the SSFP pulse sequence in the remaining phase series as substitute for the data of the high frequency regions; to reconstruct low space frequency images based on the low frequency regions to which the substitute data have been added; to reconstruct high space frequency images based on the high frequency regions to which the substitute data have been added; and to totally add the low space frequency images and the high space frequency images multiplied by a scaling factor.
In a seventh aspect, the invention provides a version of the MRI apparatus control method according to the sixth aspect, characterized in that the scaling factor is the ratio between the sum of a plurality of factors, figured out for each of a plurality of images reconstructed as the root-mean square of pixel values on the basis of the data of the low frequency regions to which the substitute data have been added, and one factor out of those factors.
In an eighth aspect, the invention provides a version of the MRI apparatus control method according to the seventh aspect, characterized in that the one factor has been figured out of an image which, out of the plurality of images, derives from data collected in the SSFP pulse sequence in the first phase series.
In a ninth aspect, the invention provides a version of the MRI apparatus control method according to the sixth aspect, characterized in that, in the low frequency regions, the matrix size in the central part, of the k-space having a matrix size of 256×256, is 64×256.
In a 10th aspect, the invention provides a version of the MRI apparatus control method according to the sixth aspect, characterized in that the plurality of phase series are four phase series.
According to the invention, it is possible to realize an MRI apparatus that permits imaging in a short length of time though using an SSFP pulse sequence in which RF excitation is accomplished by a phase cycling method.
Also, by the MRI apparatus control method according to the invention, it is possible to realize an MRI apparatus control method which permits imaging in a short length of time though using an SSFP pulse sequence in which RF excitation is accomplished by a phase cycling method.
As the scaling factor is the ratio between the sum of a plurality of factors, figured out for each of a plurality of images reconstructed as the root-mean square of pixel values on the basis of the data of the low frequency regions to which the substitute data have been added, and one factor out of those factors, the scaling factor can be made appropriate.
Since the one factor has been figured out of an image which, out of the plurality of images, derives from data collected in the SSFP pulse sequence in the first phase series, the scaling factor can be made appropriate.
In the low frequency regions, since the matrix size in the central part, of the k-space having a matrix size of 256×256, is 64×256, the low frequency regions can be made appropriate.
Since the plurality of phase series are four phase series, high quality images free from band artifacts can be obtained.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.